Generally, CCD image sensors comprise a plurality of photodetectors (such as photodiodes or photocapacitors) arranged in an array. The array may be linear in which the photodetectors are in a single line, or area where the photodetectors are arranged in a plurality of rows and columns. The image sensor also includes at least one CCD shift register. In a linear array the shift register extends along the line of photodetectors. In an area array there is a separate shift register along each column of the photodetectors vertically, and a horizontal shift register connecting all of the vertical ones. Each CCD shift register comprises a channel region and a plurality of gate electrodes over and insulated from the channel region. The gate electrodes are arranged along the channel region.
A common type of image sensor uses a two-phase shift register in which there are two sets of gate electrodes with a pair of gate electrodes (one from each set) being adjacent each photodetector. There is also a transfer means between each photodetector and its adjacent shift register by which charges generated and collected in the photodetector are transferred to the shift register. It is a common practice in a two phase shift register to provide a transfer region between a portion of each of the photodetectors and one of the electrodes adjacent thereto. Thus, charges from the photodetector are transferred to the channel region of the shift register under only one of the gate electrodes of the adjacent set of gate electrodes.
In order to increase the resolution of a CCD image sensor, it is necessary to increase the number of pixels (photodetectors) in the device. However, in order to maintain the image sensor at a relatively small size, the size of each photodetector and the spacing between photodetectors must be decreased to allow more of the photodetectors to be formed without increasing the area of the device. Decreasing the size of the photodetectors also requires decreasing the size of the related elements, such as the gate electrodes and the transfer regions between the photodetectors and the shift registers. However, decreasing the width of the transfer regions leads to considerable narrow width effects, including the requirement that the voltages on the transfer gates and the CCD gate electrodes be substantially different from each other, which results in increased system complexity.